Medyo mahirap ipaliwanag ng maayos kasi it depends on the actual harware implementation. Kung typical load balancing router with two wan ports, and each wan port gets it own ip address, hindi magdodoble yung bandwidth. Load balancing lang ang mangyayari which is typically round robin style. Limitation yan ng ip protocoal stack running at layer 3. Kung layer 2, then may tinatawag na port aggregation at yun, talagang ang bandwith ay magiging doble or bw1 + bw2.
Example, if using a single pc, lets say firefox web browser traffic will be routed to wan port1, tapos may chrome ka, the chrome traffic will be routed through wan port 2. So solong solo ni firefox and bw ni port 1 while solong solo ni chrome ang bw ni port 2.
If naka dsl ka, tapos maximum data rate mo because of the long distance eh 1.5Mbps lang, you can ask your provider if they support pair bonding so you can get double the speed. Of course the modem used and remote mux must also support pair bonding. That kind of scenario is working on the layer 2 level.